This invention relates generally to a method of producing an ionization chamber detector, and more particularly to a method of fixing electrodes of an ionization chamber detector for use in computed tomograph.
A whole body CT (compound tomograph) has a construction such as shown in FIGS. 1(a) and 1(b). A hole 21 is bored at the center of a disc 20 for the insertion of the head or abdominal region of a man, an X-ray tube 22 is mounted onto the disc 20 and an X-ray detector 23 is disposed on the disc 20 in such a manner as to oppose the X-ray tube 22. A subject is interposed between the X-ray tube 22 and the detector 23, and an X-ray beam is rotated and radiated on a predetermined plane while the disc 20 is being rotated. The detector 23 receives the beam and an image is re-constructed by a computer from the output data of the detector 23.
An ionization chamber detector for measuring the spatial distribution of X-rays has been used as the detector 23 described above.
The ionization chamber detector 23 has a construction such as shown in FIG. 2. A plurality of flat anodes 2 and a plurality of flat cathodes 3 are arranged in such a manner as to keep substantially parallel gaps between them, and a gas such as Xe gas of from about 10 to about 50 atms is then sealed.
When the X-rays are incident to the ionization chamber detector described above, they are dissociated to the xenon ion Xe.sup.+ and the electron e.sup.-, and a dissociation current is collected by applying a high voltage across both electrodes 2 and 3.
As schematically illustrated in FIG. 3, a plurality each of anodes and cathodes are alternately arranged with substantially parallel gaps between them and are bonded and fixed to upper and lower insulators 1. In other words, ditches for supporting and fixing the flat electrodes 2, 3 are formed equidistantly on the insulators 1, and the electrodes 2, 3 are inserted into, and fixed to, these ditches. The X-rays are incident as a fan beam that expands in a substantially flat arcuate shape, for example, from the direction represented by an arrow in FIG. 3. Therefore, the dissociation current generated in the space existing between the anodes 2 and the cathodes 3 is measured as described above. The spatial resolution of the detector 23 of this kind is significantly affected by the number of detection cells consisting of the anodes 2 and the cathodes 3 per unit length in the direction of the fan-shaped arrangement and by the mechanical accuracy (the flatness of the electrodes, the accuracy of locating the ditches for supporting and fixing the electrodes, and the like). Therefore, the detector 23 generally consists of more than 1,000 pairs of detection cells to improve the spatial resolution. (Refer, for example, to Japanese Patent Laid-Open No. 69491/1979.)
However, the distance between each anode and cathode 2, 3 constituting the detection cell may be less than several hundred microns. Therefore, an extremely high level of skill is necessary in order to accurately form the ditches for inserting, supporting and fixing the electrodes while keeping an accurate distance between the electrodes 2, 3, and to bond and fix them into the ditches. If the electrodes are bonded and fixed insufficiently in the ditches, a microphonic noise develops due to the vibration of the electrodes. If a difference of more than about 10 microns exists between the electrodes 2 and 3 of adjacent detection cells, a ring-like artifact will appear in the reconstructed image. Therefore, such a detector cannot be used as a detector for a CT.